To absorb vibrations of crankshaft of automobile engine, a damper is fixed to a crankshaft end. The damper comprises a hub fixed to the crankshaft end by a center bolt and an annular mass member (vibration ring) provided at the outer periphery of the hub, and a rubber elastic body through which the annular mass member is joined to the hub.
Vibration isolation mechanism of a damper is to isolate vibrations by tuning the resonance frequency of the damper to that of the crankshaft, and the resonance frequency of the damper depends on the inertial mass of vibration ring and the spring constant of rubber elastic body. Furthermore, the spring constant of rubber elastic body is temperature-dependent, so the tuning has been so far made by using a spring constant at 60° C. to absorb vibrations in the temperature region of about 20° to about 100° C. as the normally usuable temperature region.
Thus, the vibration isolation effect at temperature lower than the afore-mentioned temperature region has been outside of consideration. For example, in the case of using a damper at such low temperatures such as about −30° C., the resonance frequency of the damper is far off the frequency as tuned, resulting in insufficient absorption of vibrations, i.e. increasing vibrations of crankshaft and increasing car interior noises.
With the improvement of engine performance, on the other hand, vibration input to the crankshaft is increasing. Unless torsional vibrations of the crankshaft is reduced, the vibration will increase, lowering the safety factor of parts. Thus, a damper is used to reduce the vibrations. To cope with the increasing consumption of electric power by automobile auxiliary machinery, the effective diameter of a damper must have been enlarged. In general, the larger the inertial mass of a vibration ring, the better the vibration isolation effect of a damper. Thus, the inertial mass of the vibration ring in the damper tends to increase with an increasing output of the engine and with increasing power consumption of the auxiliary machinery. On the other hand, the smaller the total inertial mass of a damper as a whole, the less the vibrations generated per se by the engine. Thus, the total inertial mass has an optimal range.
The conventional damper can suppress torsional vibrations of crankshaft only in the normal use temperature region, but in the low-temperature region its vibration isolation function sometimes fails to be fulfilled, resulting in a risk of generation of a large vibration input in excess of the fastening torque of a bolt at the bolt-fastened position. Particularly in the case of a damper with a large total inertial mass such phenomena will occur with a high possibility.
Such phenomena, when occurred, can be overcome by circulating hot water or lowering the allowable revolution rate of the engine while the temperature of rubber elastic body in the damper is elevating by heat from the engine, but additional provision of sensors, hot water pipings and engine control program for this purpose will be required, or further problems of necessary consideration of providing additional parts at narrow positions, weight increase, cost increase, etc. will appear.
Dampers with distinguished vibration isolation effect even in low-temperature circumstances while maintaining the vibration isolation effect in the normal use temperature region are now in demand. It has been proposed to use an elastic body obtained by sulfur vulcanization of an EPDM composition, whose diene component in EPDM is 5-ethylidene-2-norbornene, in a copolymerization ratio by mole of ethylene to propylene is 60/40 to 73/27, the composition further containing a liquid-α-olefin copolymer, or an elastic body obtained by sulfur vulcanization of an EPDM composition, whose diene component in EPDM is 5-ethylidene-2-norbornene, in a copolymerization ratio by mole of ethylene to propylene is 65/35 to 73/27, in a dynamic damper having a structure comprising a vibration body, a mass member and an elastic body through which the mass member is joined to the vibration body.
PATENT LITERATURE 1: JP-A-10-89409
PATENT LITERATURE 2: JP-A-10-89410
To improve the low-temperature characteristics of polymer, the proposed attempt is to impair the crystalinity of ethylene units by increasing a proportion of propylene unit, thereby making the low-temperature characteristics better and also to make the temperature dependency of rubber material better by increasing the molecular weight.
However, in the case of simple use of such EPDM polymers the glass transition point Tg shifts toward the lower temperature side by making the propylene proportion higher, and consequently the tan δ peak shifts toward the lower temperature side, so the damping characteristics of rubber material is inevitably lowered considerably. The temperature dependency of spring constant can be made better by increasing polymer molecular weight, but the damping characteristics are inevitably lowered thereby.
Thus, in the case of simple use of such EPDM polymers as a rubber member for the damper, the damping characteristics are lowered in the normal use temperature region and the strain is increased at the resonance time, though the low-temperature characteristics can be improved. As a result, the durability, etc. will be adversely affected. Furthermore, the improvement of low-temperature characteristics by polymers has a limit due to the copolymer composition, as mentioned above, and thus further improvement of the low-temperature characteristics is hard to attain.